: MHC class II (Ia) molecules on microglia is a common hallmark of many demyelinating diseases including Alzheimer's disease, Huntington's disease, Parkinson's disease and multiple sclerosis. We propose to study microglial activation in two murine models of demyelination, the twitcher mouse which is a model for Krabbe s disease and cuprizone treatment of mice which is a neurotoxin-induce demyelination. The purpose of this proposal is to determine mechanisms by which MHC class II molecules activate microglia and the consequence of the microglial activation on neuropathology. The Specific Aims are threefold: 1). Assess if Ia molecules mediate signal transduction to activate microglia in vivo. The mechanism by which Ia exacerbates demyelination in the twitcher model is not clear. We will determine if signal transduction via the I-Ab chain is responsible for the activation of microglia and the subsequent increased severity on demyelination. We have obtained mice with a I-Ab-truncated cytoplasmic tail in which the signaling domain has been removed. These I-Ab truncated mice will be mated to twitcher mice to produce I-Ab-truncated twitcher mice and these mice will be compared to wildtype twitcher mice for clinical symptoms, neuropathology and microglial involvement. 2). Determine whether the cytoplasmic domain of the Ia molecule functions in signal transduction in microglia in vitro. MHC class II molecules can serve as signal transduction molecules on other cell types; however, it would be important to demonstrate this in microglia. We will us structure and functional analyses to test which domain of the I-Ab chain is responsible for microglial activation. In addition, we will use primary microglia from I-Ab-truncated, I-Ab-/- and wildtype mice and test whether the presence or absence of engaged MHC class II molecules affect microglia activation in vitro. 3). Determine whether T-lymphocytes play a role in neurodegeneration in the twitcher mice. This Aim will address the issue of T cell involvement in the twitcher mouse by using the RAG-1 knockout mice. Although the twitcher mice exhibit little apparent T cell infiltration, others have shown that few activated T cells in the CNS may still play an important role. We will directly assess the role of T cells by mating twitcher mice to the RAG-1-deficient mice which are devoid of mature lymphocytes and determine outcome of demyelination. In addition, we will adoptively transfer splenic T cells from twitcher mice into RAG-1-deficient twitcher mice and test if T cells reconstitute the severity of demyelination.